Cermet electrical resistance material and method of using the same



Sept. 26, 1967 R. c. vlcKERY 3,343,985

CERMET ELECTRICAL RESISTANCE MATERIAL AND METHOD OF USING THE SAME Filed Feb. 12, 1963 y 4 sheets-sheet 2 ,DOM/E'FEO 62455 MET/4L PES5/4 TE SOLUT/O/V HFA T 70 DI? V D DECO/VRQSE PES/NA 7'5 CAL C//V GIF/N0 T0 100M/0E EEGEEGA TE pon/DEE vra @POL/'ps Acfo/A/c: To @wr/@E s/ZE.

@Mama/crm@ w+ fr0/QM AYER Foe/v EA CH EASE wscous M/x TUBE o/v A BASE C7195v EACH LA VEP ABVE MELT/NG DO/N7' OF GLASS AND BELOW MELT/NG DO/N7- 0F' METAL JELECT /VE DE MPE GROUPS ELEC TE/CAL CHARACTER/ 77C OES/@E0 l/V CER/VE 7' /PES/SZ'QNC'E ELEMENT' INV TOR.

/CKEP V BY #Z914 5,? Af/)0885 .ia/wao rrof/VEVE K Sept. 26,l 1967 R. c. vlcKERY 3,343,985

CERMET ELECTRICAL RESISTANCE MATERIAL AND METHOD OF USING THE SAME 4 ,Sheets-Sheet 4 Filed Feb. 12, 1963 INVENTOR .IPO/mm C Wm/5er BY Fon/Ee Nasse United States Patent O 3,343,985 CERMET ELECTRICAL RESISTANCE MATERIAL AND METHOD F USING THE SAME Ronald C. Vickery, Saxonburg, Pa., assignor to Beckman Instruments, Inc., a corporation of California Filed Feb. 12, 1963, Ser. No. 257,957 Claims. (Cl. 117-227) The present invention relates to an improved cermet electrical resistance material and methods for making same.

Cermet resistance elements presently known in the art are exemplified by U.S. Patent 2,950,995 of Thomas M. Place, Sr. et al., entitled Electrical Resistance Element, and 2,950,996 of Thomas M. Place, Sr. et al., entitled Electrical Resistance Material and Method of Making Same, both of which are assigned to Beckman Instruments, Inc., assignee of the present invention. These patents describe resistance elements formed by a layer of resistance material comprising a heterogeneous mixture of non-conducting material and conducting metals fixed to a fuse. The non-conducting material is a ceramic type material such as glass and the layer is formed by heating the metal-glass mixture at least to the melting point of the ceramic material but not to the melting point lof the metals, so as to create a smooth, glassy phase. Additional prior art directed toward resistors formed of a glass-metal composition are U.S. Patent No. 2,837,487 of Daniel E. Hnttar, entitled Resistor Enamel and Resistor Made Therefrom, and U.S. Patent No. 2,924,540, of I ames B. DAndrea, entitled Ceramic Composition and Article.

Cermet resistance elements constructed according to the aforementioned Place et al. patents have very good electrical characteristics, e.g., a predetermined ohmic resistance per square, a low temperature coefficient of resistance, low noise, and high resolution. Moreover, the ceramic nature of these elements permits them to be operated in very high temperature environments. These features have each contributed to the use of these cermet resistance elements both as fixed resistors and in miniature potentiometers, particularly of the type shown in the copending applications of I ames F. Gordon, Ser. No. 584,088 and now abandoned, entitled Potentiometer and Method of Manufacturing Same, filed May 10, 1956, Jack E. Langenbach et al., Ser. No. 166,054, entitled Improved Variable Resistor, filed Jan. 15, 1962 (now abandoned) and William l. H. Thoele, Ser. No. 166,199 (now Patent No. 3,178,664), entitled Variable Resistance Device, led I an. 15, 1962, all of which are assigned to Beckman Instruments, Inc.

Although the teachings of the above-identified Place et al. patents may be employed to produce electrical resistance elements having very good electrical characteristics, a problem encountered heretofore is that the resistance elements have not always been consistently reproducible as to one or more of the characteristics enumerated above. This lack of consistent reproducibility has necessitated a careful testing of the resistance elements land the rejection and resultant loss of those elements not conforming to predetermined specifications.

It is, therefore, an object of the present invention to provide consistently reproducible cermet resistance elements having excellent electrical characteristics, eg., a predetermined ohmic resistance per square, a low temperature coefiicient of resistivity, and low noise and high resolution.

Another feature of this invention is to provide improved methods for constructing cermet resistance mate- ICC rial, and in particular, resistance elements having even lower temperature coefiicients of resistivity, lower noise and higher resolution than elements `constructed according to the prior art methods.

Other and further objects, features and advantages of the invention will become apparent as the description proceeds.

In brief, this invention may be summarized as being directed toward the discovery that for a given composition of glass and metal, the electrical characteristics may be substantially modified and improved by segregating the cermet material according to particle size into a plurality of narrow ranges and using one or more selected ranges for constructing the cermet resistance elements. Another aspect of this invention is directed toward the finding that the cermet material may be substantially modified and improved by selecting glass compositions having predetermined viscosities.

A more thorough understanding of the invention may be obtained by a study of the following detailed description taken in connection with the accompanying drawings in which:

FIG. 1 is an isometric view of an embodiment of the invention which is suitable for use in rotary potentiometers;

FIG. 2 is an isometric view of another embodiment of the invention which is suitable for use in linear potentiometers as well as for fixed resistors;

FIG. 3 illustrates the Gaussian or normal distribution used to define the particle groupings used in the methods of this invention;

FIG. 4 is a ow diagram of one method of the invention;

FIG. 5 is a flow diagram of another method of the invention;

FIG. 6 is a flow diagram of still another method of the invention;

FIG. 7 illustrates the resolution deviations typically obtained from a cermet resistance element; and

FIGS. 8a, 8b, 8c and 8d illustrate cross-sectional views of the resistance layer during its formation with particles of several sizes (FIGS. a, b and c) and particles of substantially uniform particle size (FIG. 8d).

Structures of represenatve resistance elements In the structure of 1FIG. l, a layer 10 of resistance material is fired to a base 11 with electrodes 12, 13 being provided at each end of the layer 10 for connecting into an electrical circuit. This resistance element may be used as a fixed resistor or may be combined with a rotatable contact arm for use as a rheostat or potentiometer. The base 11 may be of any suitable electrically non-conducting material which will withstand the elevated temperatures normally used to fire the resistance material. Various ceramic materials are suitable for this use, those having a smooth, fine textured surface and being impervious to moisture and other liquids being preferred. Steatite, fosterite, sintered or fused aluminas and Zircon porcelains are examples of preferred materials for forming the base 11.

The electrically conductive electrodes 12, 13 are conventional and may be formed by applying any of the wellknown conducting silver or other metal pastes over the layer of resistance material and firing the unit to convert the paste to a layer of metal which is firmly attached to the layer of resistance material. Alternatively, terminal structure such as is shown in the co-pending application of Kenneth F. Miller et al., Ser. No. 217,689 now Patent 3 No. 3,134,085, entitled Improved Terminal Structure for Resistance Elements, may be employed for making electrical contact with the resistance layer 10.

FIG. 2 illustrates another form of the resistance element of the invention in which a layer of resistance material is applied to a rectangular base 16 and eletrodes 1.7, 18 are then added to the layer 15. This form of the invention is particularly suitable for use as a xed resistor or linear potentiometer.

Prior art methods of forming cermet resistance elements U.S. Patents Nos. 2,950,995 and 2,950,996 teach methods of preparing the cer-met resistance layer 10. A preferred method taught therein comprises mixing the resinates of one or more noble metals. The glass binder, in the form of very small glass particles, is mixed or milled with the resinate solution so that each glass particle is thoroughly wetted with the metal solution. This mixture is gradually heated to approximately 700 F. and constantly stirred, to remove the volatiles and organic materials from the mixture, to decompose the metal compounds and to oxidize the oxidizable metals. The resulting dry material is ground to a fine powder and calcined at about 850 F. The resulting calcine is ground to a fine powder, producing a dry material consisting of very small glass particles coated with an extremely thin layer of metal particles.

Another method of preparing the resistance material is forming small particles of a desired noble metal alloy by preparing an admixture of resinates of the individual noble metals. The solvents are iirst driven olf by subjecting the mixture to a low temperature, e.g., 350 F., followed by pyrolysis or thermodecomposition of the organic material. Under normal circumstances, melting together of the metals at high temperatures, e.g., 3500" F., would be necessary for alloy formation. However, because of the very small, virtually atomic-size metal particles produced by resin pyrolysis, alloy or solid formation takes place at the relatively low temperatures of 400- 600" F. The resulting metal powders are then closely associated by a mixing or milling step with finely divided glass particles. This mixture may be further subjected to a calcining step at about 850 F. At this temperature, the glass is slightly softened so that the metal particles attach to the glass particles. The calcine material is then ground to a line powder.

The mixtures formed by the methods described in the preceding paragraphs may be stored indefinitely and may be used in small portions to produce limited numbers of resistance elements. When it is desired to make resistance elements using the material, the dry powder is mixed with a suitable liquid carrier to form a iiuid composition which can be applied to a non-conductive base material. The base with the layer applied thereto is then fired to produce a continuous phase of solidified glass. Representative tiring procedures and temperatures are taught by the Place patents and may be used to re the materials formed by this invention.

Particle size segregation of cermet resistance material The prior art literature has speciiied in particularly the percentages of glass and metal alloy required to produce useful electrical resistance elements. As noted heretofore, these elements have not always had consistently reproducible electrical characteristics. The present invention relates to the discovery that, for a given composition of metal and glass, the electrical characteristics may be substantially modified and improved by limiting the cermet material particle size to one or more very narrow ranges. In particular, reproducibility of the ohmic resistance per square, the temperature coefcient of resistivity, and contact noise and resistance may be substantially improved.

Extremely small particles may be segregated as to size by techniques based upon Stokes law, which states that spheres falling through a viscous medium attain a velocity which is directly proportionate to the square of their radii. Particles may thus be separated as to size by allowing them to fall through a viscous medium in a settlin-g column. The parti-cle size may then be related to the time required for them to fall the length of the col-umn, the lar-ger particles reaching the bottom first and the smallest particles last. A preferred particle segregation technique e-mploys air elutriation. Commercial machines are available for segregating mixtures of very line particles, eg., particles having a mean size of 1A micron and a range of less than one micron. A representative machine is the No. 6000 Micro-Particle Classifier manufactured by the Harry W. Dietert Company of Detroit, Michigan.

The particle groupings used in the methods of the invention may be conveniently dened by statistical terminology. If each group is considered as a normal or Gaussian distribution of particles, the group will have a mean or average particle diameter The particles above and below this mean particle size differ from the mean value by their individual deviation x. The variance or mean square deviation is derived by squaring each of the deviations, summing these squared deviations to obtain 2x2 and dividing by the total number N of particles in the group to obtain 2x2/N. The variance is also known as the mean square deviation. The square root of the variance is termed the standard deviation (s) s\/Ex2/n (1) For a group of particles arranged according to a normal distribution (FIG. 3), 68.27% of the particles are included in the range is, 95.45% are included in the range iZs, and 991.73% are included `in the range iSs. An additional term which is useful for comparing different groups of particles is the measure known as relative dispersion (V) which for any group is the standard deviation divided by the mean value or The air elutriation method of particle size segregation may be used for preparing several particle groupings having a relative dispersion in the order of l5 to 25 percent. A representative group is one having a mean particle diameter of 2 microns and a standard deviation s of .33 micron. Its relative dispersion V is thus 16.5%. Other representative groups prepared from a standard 1A micron to 40 micron group of particles are tabulated in Table I.

Methods of preparing cermet resistance material according to this invention In the How diagram of FIG. 4 is shown one embodiment of the method of preparing the resistance material of the invention in which the resinates of one or more noble metals are milled with very small glass particles. The mixing is carried out thoroughly so that each glass particle will be wet with the metal solutions. This mixture is gradually heated to approximately 700 F. and constantly stirred, to remove the volatiles and organic materials from the mixture, to decompose the metal compounds, and to oxidize the oxidizable metals. The resulting dry material is ground to a tine powder and calcined at about 850 F. The resulting calcine is ground to a tine powder so that each of the particles is less than 40 microns. This powder is then segregated according to particle size in the manner hereinabove vdescribed so as to produce relative groups su-ch as are shown in Table I. In a variant of this method, an initial segregating step may be performed as regards the glass particles. The segregated groups of particles are then separately wetted with the metal solutions, and the resulting metal coated glass particles again segregated according to particle size.

Each of these -groups of segregated particles is then mixed with a suitable volatile liquid carrier to form viscous mixtures and applied to a non-conductive base. The base with a layer applied thereto is then tired to produce a continuous phase of solidied glass.

One or more electrical characteristics of each of the resulting resistance elements are then carefully measured, these characteristics including resistance per square, temperature coefficient of resistivity, noise, and resolution. With this data, one or more groups may be selected for use in the production of a number of resistance elements. For example, in some instances it will be noted that a particular group may give a negative temperature coefiicient of resistivity whereas another group may give a positive coefficient of similar magnitude, The combination of these two groups will produce a resultant resistance material in which the coefficients of resistivity will tend to cancel one another, thereby providing a substantially reduced temperature coefficient.

In another method of preparing the resistance material of the invention shown in the ow dia-gram of FIG. 5, a powder composed of small particles of a predetermined metal alloy is prepared. The technique for preparing this powder described above (in which an admixture of resinates of the individual noble metals is subjected to a low temperature) is preferred because of the low ternperatures required for alloy formation. The resulting metal powders are then segregated according to particle size in the manner -described above into representative groups such as are tabulated in Table I. Each of these groups of metal powder is then closely associated by a mixing or milling step with nely divided glass particles. The resulting metal-glass mixtures are then mixed with a suitable carrier which is applied to a non-conductive base. The base is then fired to produce a continuous phase of solidified glass. Each of the resulting resistance elements is then measured in the mzmner of the previous method to determine one or more electrical characteristics, e.g. the resistance per square, temperature coeicient of resistivity, noise and resolution. The group or groups of metal particles which provide the desired resistance element or which may be combined to provide the desired element are then utilized for producing the production run of resistance elements.

It has been discovered that within a given metal-glass composition, the resistance per square, the temperature coefficient of resistivity, and other electrical characteristics differ widely according to the `groups of particle sizes. It is believed that this accounts for the lack of reproducibility noted for the prior art cermet resistance materials. The specific quantities of particles of a given size have been heretofore primarily a matter of random selection since the grinding techniques such as ball milling do not provide consistent groupings of particle sizes. Stated in another way, one batch of material produced according to the prior art method could predominate in particle sizes `between 1A and 25 micron-s. As will become apparent hereinafter, the inadvertent inclusion of a substantial number of a particular-sized particles will strongly affect the electrical characteristics of the resulting resistance elements.

Representative examples The improvements in cermet resistance elements which Ymay be achieved by the methods of this invention are further illustrated by reference to a specic metal-glass mixture. While innumerable combinations of materials may be used in fabricating cermet resistance elements of the invention, the following are set forth as being representative thereof:

Example I Particles of a material having the composition:

Percent Glass 92.50 CuSn Material (%:5%) 2.50 Gold 2.50 Palladium 1.95 Calcine .55

were prepared utilizing the teachings of U.S. Patent No. 2,950,996.

These particles were then graded into various particle sizes in accordance with Table I above and each group used to make a cermet resistance element. The resistance in ohms per square and temperature coeicient of resistivity in parts per million per degree centigrade for each of these elements is given in Table II.

TABLE II Resistance Temperature coeI- X (microns) (ohms/square) cient of Resistance (p.p.m./ C.)

These individual values for the different particle size groups may be compared with the resistance element produced from the unsegregated material which has a resistance of 2320 ohms per square and a temperature coecient of resistivity of 39.2 p.p.m./ C. The groups of particles having a mean particle size of 0.25, 3 and 5 microns were used to make resistors having a resistance of 2610 ohms per square and minimum temperature coefficient of resistivity. The resulting resistance elements allowed a much better control of resistance and temperature coefficient of resistivity than for elements made with the unsegregated material. The 1, 2, 8 and 12 micron mean particle size groups were found to be suitable for producing resistance elements having a resistivity of 1305 ohms per square. In particular, it was found desirable to remove the particle grouping having a mean particle size of 0.5 micron since this particular grouping has a very large negative temperature coeicient of resistivity. It is apparent from Example I that inadvertent inclusion of different portions of the 0.5 micron particle sizes will result in quite inconsistent temperature coefficients of resistivity.

Example 2 Particles of a material having the composition:

Percent Glass 68.06 CuSn Material (95% :5%) 5.92 Silver 13.32 Palladium 3.45 Gold 9.21

were prepared utilizing the teachings of the Place et al. patents, supra.

Particles of this material were then graded according to particle sizes in accordance with Table I and each group used to make a cermet resistance element. The resistance and temperature coefficients of resistivity for each of these elements is given in Table III.

TABLE III Resistance (ohms/square) Temperature coefficient of Resistance (microns) (p p m lo C Non-conducting N on-conducting N ori-conducting These values may be compared with the resistance element produced from the unsegregated material which had a resistance of 7.85 ohms per square and a temperature coeicient of resistivity of +350 p.p.m./ C. In this instance, the desired resistance had a lower resistance in the order of 1.3 ohms/square. By removal of the liner particle groups, i.e., those having a mean particle diameter of 1/2, 1 and 2 microns which were non-conducting and also the 3 micron size which had a larger resistance than desired, a composition which was previously unusable for low resistance values was made quite satisfactory therefor. In addition, this material had the added advantage of uniform resistance and temperature coeiiicient of resistivity.

Example 3 Particles of a material having the composition:

Percent Glass 95.5 Gold 2.7 Palladium 1.8

were prepared utilizing the teachings of U.S. Patent No. 2,950,995.

These particles were then graded according to particle size and each group was used to make a cermet resistance element. The resistance and temperature coeflicients of resistivity of each of these elements is given in Table IV.

TABLE IV Resistance Temperature coerh- X (microns) (ohrns/square) cient, of Resistance (p.p.m./ C.)

0. 5 N on-conductive 6.5 42, 000 -700 12 Non-conductive Example 4 Particle groupings substantially larger in range than those shown in Table I have been found to sometimes show a substantial change in electrical characteristics for particular glass-metal compositions. By way of specic example, particles ranging in size from less than a quarter micron to over fty microns were prepared having the composition:

Percent Glass 75 Iridium 18,75 Gold 6.25

These particles were prepared according to the teachings of the co-pending application of Donald A. Bruhl, Jr. et al. entitlted Improved Cermet Resistance Material, Ser. No. 258,056, iiled on even date herewith and assigned to Beckman Instruments, Inc. These particles were then graded into three particle groupings ranging between less than 0.25 and 2 microns in diameter, between 2 Inicrons and 35 microns in diameter, and between 35 and 50 microns in diameter. The resistance and temperature coeiiicients of resistance elements constructed from materials selected from these ranges are tabulated in Table V.

A resistance element having a resistance in the order of 3200 ohms per square and a reasonably low temperature coeicient was constructed using the particles within the range of 2 to 35 microns and excluding those less than 2 microns in diameter and greater than 35 microns in diameter.

Improvements in resolution Particle size segregation has also `been found to sub- -stantially improve the resolution of the resistance elements, particularly when the smaller particle sizes are utilized therefor. An illustrative example is as follows:

Example 5 Particles of a material having a composition:

Percent Glass Iridium 11.25 Gold 3.75

were prepared land `graded to particles whose diameters were less than one micron, between two and live microns, and between ten and 35 microns. Resistance elements were then constructed from each group of particle sizes and then mounted in combination with a movable electrical contact. The deviation of the output from a perrfectly straight line was then plotted and produced the type of curve shown in FIG. 7. The deviation from this curve was then measured as a percentage of the total applied voltage, these percent deviations for the three different particle size groups being tabulated in Table VI.

TABLE VI Resolution (Deviation) Particle size group: Percent Less than 1 micron i025 2-5 microns 1.04 10.35 microns $0.10

The same material without particle size segregation has a deviation of i0.1 to i.4%.

Modz'cations and improvements aolded by preselectz'on of glass viscosity The particular composition glass utilized in the foregoing examples is not critical, a representative composition being given in U.S. Patent No. 2,950,996. Another aspect of this invention, however, relates to the discovery that for a given glass-metal composition and tiring temperature, the electrical characteristics of the resulting resistance element may be substantially modified Iand improved by preselecting a glass composition having a predetermined viscosity. More particularly, it has been found that increasing the glass viscosity changes the temperature coei'licient of resistance in a positive direction, whereas decreasing thc glass viscosity changes the temperature coeflicient of resistivity in a negative direction. One improved method, therefore, of constructing cermet resistance elements having a predetermined temperature Illustrative examples showing the effect upon resistance and particularly upon temperature coeicient of resistivities are as follows:

Example 6 phase.

coetiicient of resistivity is shown in the iiow diagram of FIG. 6 and includes constructing a iirst celrmet element Particles of a material having a composition:

of a predetermined glass and metal composition followed by a' measurement of its temperature coeiiicient of re- Percent sistivity. Another `glass composition having a viscosity Glass 90 selected in accordance with the desired temperature co- 10 Iridium 9.09 efficient is used to construct another cermet element. Its Gold .91 temperature coecient is also tested and it satisfactory, t the selected glass is used; otherwise still another glass Were Pnl-Pared 'UtlllZlflg tW0 dlifel'f glass COIHPOSIUOHS, composition is selected in accordance with the viscosity- O11@ eXamPlo em-POylIlg 011e part 0f GlaSS t0 ten parts temperature coefficient relationship noted above. 15 0f Glass B, and the other @Xample employing one part In the above described method, the proportions of glass 0f Glass B l0 ihre@ Parts of Glass C The former, COH- and metal may remain Constant 0ithey '[00 may vary, s tructed with the lower viscosity glass provided a re- Thus, the temperature coefficient of resistivity normally SISanCo element havlng a reSlStanCe of 207,000 ohms per increases positively with increased metal content. With Square and a temperature cceicient of resistivity of the methods yof this invention, however, the temperature w690 PPH1- C The latter glaSS, COIISIUCod With the coecient of resistivity may be maintained at a desired higher viscosity glass, provided a resistance element havvalue by selecting a glass of decreased viscosity for higher 111g a resistance of 34,300 ohms per square and a submetal content and a glass of increased viscosity for lower Stantially lower (more positive) temperature coeicient metal content. of resistivity of i-9l p.p.m./ C.

Representative examples of glasses A, B and C having Additional examples showing the positively increased respectively low, intermediate and high -Viscosities at a temperature coefficient of increased viscosity are tabugiven ring temperature are as follows: lated in Table VII.

TABLE VII Glass Temperature Example Metal Resistance Coecient of No, (ohms/square) Resistivity Percent of Composition (p.p.m./ C.)

Total 1 part Glass A 11. 25%

to Iridium 26, 000 -3. 4 9 parts Glass B 3. 75% 7 a5 Palladium 1 part Glass A i0 14,000 51.0 1 part Glass B l part Glass A 14. 2% 8 84 to Iridium 14, 000 o 9 parts Glass B 1. 8% Gold 1 part Glass A 17. 8% 9 so to Iridium 7, ooo o 9 parts Glass B 2. 2% Gold x Examples 8 and 9 particularly illustrate the control Glass A'(L0w Vlscoslty) over the temperature coeiiicient which may be achieved Fused composition (percent) with the preselected glass viscosities, these examples being Lead Oxide 72.15 a gold-iridium composition constructed in accordance with Zinc oxide 5.40 the cio-pending application of Donald A. Bruhl et al., Boric oxide 9.04 supra, and the metal content is changed from over 16% Silica 13.41 to 20%. Ordinarily, with increasing metal content, the Glass B (lntemedate Viscosity) temperature coeiiicient or' resistivity becomes more posii tive. However, the expected increase of temperature c0- Fused composition (pernU eiiicient is obviated in the example shown by decreasing Lead Olde -68 6() the glass viscosity. The temperature coeicient resistivity Z111? GX10@ 5-41 has therefore been maintained at zero, although identical BOflc OXlde 10-00 methods and tiring temperatures were used for both 0f Silica 16-51 the examples. Zirconium oxide 2.4

Gl' C H h V 65 Combined particle size and glass viscosity selection ass i scosit g y Further improvements in cerrnet material are aiorded Fused composition (percent): by broth segregating the material into several groups hav- Lead oxide 60.87 ingr diierent mean diameters and selecting one or more Zinc oxide 5.02 grOUpS EO fOrm 'the resistance element, and selecting a Boric oxide 11.97 glass to -forrn the cermet material which has a predeter- Silica 21.11 mined viscosity characteristic. zirconium oxlde u 102 T heOries postulated for explaining results described Other viscosity characteristics may be obtained with heemabove other materials such as plastic resins having a glossy Although the exact reasons for the modified and irnproved electrical characteristics Which may be obtained in cermet resistance elements by particle size segregation and preselection of glass viscosity are not known at this time, this inventor has postulated the following theories for explaining these results.

During the construction `of a cermet resistance element from the fine particles of cermet resistance materials, a close admixture of metal and glass particles is initially placed -upon a nonconductive base by, for example, normal silk screening techniques. After drying to remove volatile materials, the whole system is heated to melt the glass. There is then formed a dispersion of metal particles; however, so long as the metal particles are dispersed, the system will conduct little or no electric current.

Normally, one would expect that the metal particles, being of higher density than the liquidglass in which they are now suspending, would sink to the bottom of the glass layer, the larger particle-s settling first and the finer particles later, according to Stokes law. Instead, the surface tension forces existing between the metal and the glass cause the particles to rise towards the glass/air interface at a speed controlled by the viscosity of the glass and the particle size of the metal particles. The film thickness and particle size, in turn, are believed to substantially determine the temperature ooeicient of resistivity and the resistance of the resistance layers for a given metal or metal alloy.

For example, the particle size determines the manner in which the particles are packed adjacent one another and thus the resistivity of the resulting resistance layer. Also, the thickness of the conductive layer plays an important part in determining the resistance of the layer. In a solid metal, electrons normally travel through the mean free path bef-ore striking a scattering atom; in a thin layer, however, the film surface also acts as a scattering medium.

The temperature coefl'icient of resistance of bulk metals depends upon the increase in number of electrons with increase in temperature and the increase in lattice bonding vibrations with temperature. In normal metals, the latter yfactor predominates, leading to an increased resistance with increase in temperature, i.e., a positive coei cient. In the thin layered cermet elements, the bond vibration is offset by the relative increase in availability of surface, or skin paths through which the electrons can pass without scattering by interatomic bonding vibrations.

The improved resolution afforded with particle size segregation is also explained by the foregoing theory. Referring to FIG. 8, FIG. 8a illustrates the initial formation of a film containing many different sized particles 20, 21, 22, and 23. As shown therein, the particles are dispersed throughout the glass as soon as the glass melts. As may be vseen in FIGURE 8b, the smallest particles oat first to the glass surface. Succeedingly larger particles 21, 22 and 23 are then still lioating upwards. FIG. 8c shows the resistance layer after all of the particles have attained the surface. Such an arrangement is extremely difiicult to control since the largest particles push everything else yout of the way when they attain this conductive zone. By the time this occurs, however, the lirst floated liner particles have agglomerated and moved away from each other. The resultant iilm layer is irregular and inconsistent in resistance thereby providing poor electrical resolution. If, however, the metal particles are all within a very narrow range of particle size such as the particles 24 of FIG. Sd, the final metal iilm layer is formed within a very short time interval. The formed layer is therefore quite regular and has a high electrical resolution and higL consistency of resistance throughout the entire resistive layer. These qualities are, moreover, quite reproducible from one layer to another.

Although exemplary embodiments of the invention have been disclosed and discussed, it will be understood that other applications of the invention are possible and the embodiments and methods disclosed may be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention.

I claim:

1. A method for varying the metal content of a given cermet element including at least one metal and a glass having a predetermined viscosity without varying the temperature coeliicient of resistivity of the resistance elements constructed therefrom, said method comprising the steps of:

selecting a glass having a viscosity characteristic inverse to the desired change in metal content, and

forming particles of cermet material including said selected glass having substantially the same metal -composition as the original cermet material but with the changed proportion of metal to glass,

a glass having increased viscosity being selected to provide a more positive temperature coeicient of resistivity and a glass having decreased viscosity being selected to provide a more negative coeflicient of resistivity.

2. A resistance material for use in a resistor or the like comprising:

particles of glass coated with a noble metal -or noble metal alloy, said particles being selected to have a predetermined mean diameter substantially less than 50 microns and a relative dispersion less than 25%.

said glass being selected to have a predetermined viscosity, an increased glass viscosity being selected to provide a more positive temperature c-oeicient of resistance and a decreased glass viscosity being selected to provide a more negative temperature coeicient of resistance.

3. A resistance material for use in a resistor or the like comprising:

particles yof cermet material comprising a mixture of glass and metal particles, said particles selected to have a predetermined mean diameter less than 50 microns and a relative dispersion less than 25%,

said glass of said cermet material selected to have a predetermined viscosity for achieving a desired temperature coeflicient of resistance, an increased glass viscosity being selected to provide a more positive temperature coeliicient of resistance and a decreased glass viscosity being selected to provide a more negative temperature coeflicient of resistance.

4. A resistance material for use in a resistor or the like which is consistently reproducible as to ohmic resistance per square and temperature coefiicient of resistivity comprising:

a particle group of cermet material comprising a mixture of glass and metal particles selected to have a predetermined mean diameter substantially less than 50 microns and a relative dispersion less than 25 5. A resistance material for use in a resistor or the like which is `consistently reproducible as to ohmic resistance per square and temperature coefficient of resistivity comprising:

a particle group of cermet material comprising glass and at least one noble metal selected to have a predetermined mean diameter substantially less than 50 microns and a relative dispersion less than 25%.

References Cited UNITED STATES PATENTS 2,924,540 2/ 1960 DAndrea 252--514 X 2,950,996 8/1960 Place et al. 117-227 3,052,573 9/1962 Dulnesnil 117-227 X 3,149,002 9/1964 Place et al. 117-227 3,154,503 10/1964 Janakirama-Rao 252-514 3,207,706 9/1965 Hoffman 117-227 X ALFRED L. LEAVITT, Primary Examiner.

WILLIAM L. JARVIS, Examiner.

UNITED STATES PATENT oEEIcE CERTIFICATE OE CORRECTION Patent No. 3,343,985 September 26, 1967 Ronald C. Vickery It is hereby certified thai'I error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column l, line 22, for "fuse" read base line 43, strike out "and now abandoned".

Signed and sealed this 7th day of January 1969.

(SEAL) Attest:

EDWARD J. BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

1. A METHOD VARYING THE METAL CONTENT OF A GIVEN CERMET ELEMENT INCLUDING AT LEAST ONE METAL AND A GLASS HAVING PREDETERMINED VISCOSITY WITHOUT VARYING THE TEMERATURE COEFFICIENT OF RESISTIVITY OF THE RESISTANCE ELEMENTS CONSTRUCTED THEREGROM, SDAID METHOD COMPRISING THE STEPS OF: SELECTING A GLASS HAVING A VISCOSITY CHARACTERISTIC INVERSE TO THE DESIRED CHANGE IN METAL CONTENT, AND FORMING PARTICLES OF CERMET MATERIAL INCLUDING SAID SELECTED GLASS HAVING SUBSTANTIALLY THE SAME METAL 